A storage is computer-readable media capable of storing data in blocks. Storages face a myriad of threats to the data they store and to their smooth and continuous operation. In order to mitigate these threats, a backup of the data in a storage may be created to represent the state of the source storage at a particular point in time and to enable the restoration of the data at some future time. Such a restoration may become desirable, for example, if the storage experiences corruption of its stored data, if the storage becomes unavailable, or if a user wishes to create a second identical storage.
A storage is typically logically divided into a finite number of fixed-length blocks. A storage also typically includes a file system which tracks the locations of the blocks that are allocated to each file that is stored in the storage. The file system also tracks the blocks that are not allocated to any file. The file system generally tracks allocated and unallocated blocks using specialized data structures, referred to as file system metadata. File system metadata is also stored in designated blocks in the storage.
Various techniques exist for backing up a source storage. One common technique involves backing up individual files stored in the source storage on a per-file basis. This technique is often referred to as file backup. File backup uses the file system of the source storage as a starting point and performs a backup by writing the files to a destination storage. Using this approach, individual files are backed up if they have been modified since the previous backup. File backup may be useful for finding and restoring a few lost or corrupted files. However, file backup may also include significant overhead in the form of bandwidth and logical overhead because file backup requires the tracking and storing of information about where each file exists within the file system of the source storage and the destination storage.
Another common technique for backing up a source storage ignores the locations of individual files stored in the source storage and instead simply backs up all allocated blocks stored in the source storage. This technique is often referred to as image backup because the backup generally contains or represents an image, or copy, of the entire allocated contents of the source storage. Using this approach, individual allocated blocks are backed up if they have been modified since the previous backup. Because image backup backs up all allocated blocks of the source storage, image backup backs up both the blocks that make up the files stored in the source storage as well as the blocks that make up the file system metadata. Also, because image backup backs up all allocated blocks rather than individual files, this approach does not necessarily need to be aware of the file system metadata or the files stored in the source storage, beyond utilizing minimal knowledge of the file system metadata in order to only back up allocated blocks since unallocated blocks are not generally backed up.
An image backup can be relatively fast compared to file backup because reliance on the file system is minimized. An image backup can also be relatively fast compared to a file backup because seeking is reduced. In particular, during an image backup, blocks are generally read sequentially with relatively limited seeking. In contrast, during a file backup, blocks that make up individual files may be scattered, resulting in relatively extensive seeking.
One way to accomplish image backup is using a snapshot, which enables the state of the source storage at a particular point in time to be captured without interrupting other processes, thus avoiding downtime of the source storage. Many snapshots employ a “copy on write” methodology which requires that every write command, received by the source storage during a snapshot operation, be delayed until the original data block at the location targeted by the write command is copied for safekeeping to a new location. In this manner, the copied original blocks stored in the new location, as well as the unchanged original blocks stored in the source storage, are “frozen” at the snapshot time and define the “snapshot,” which can then be employed in the creation of an image backup of the source storage. Then, once the image backup has been created, the data blocks that were copied as part of the snapshot can be discarded.
One common problem with using a snapshot during the creation of an image backup is that the source storage may not be in an ideal state at the snapshot time. For example, a snapshot may be created before an application has saved data to the source storage that would be useful during a restore of an image backup. This useful data will therefore not be included in the snapshot and therefore be missing from the image backup that is created based on the snapshot.
To mitigate a source storage not being in an ideal state at a snapshot time, post-snapshot quiescence technology has been developed to modify a snapshot to place the snapshot in an improved state for the creation of an image backup. For example, some platforms may permit software applications to issue write operations to the snapshot after the snapshot has been created. This enables these software applications to perform post-snapshot quiescence writes to the snapshot before the snapshot is used in the creation of an image backup. However, since each successive image backup in an image backup chain typically depends on prior image backups representing the exact state of the source storage at the snapshot time, and since post-snapshot quiescence technology causes a snapshot to no longer represent the exact state of the source storage at the snapshot time, post-snapshot quiescence technology is not typically employed in creating an image backup chain. Therefore, individual image backups in an image backup chain lack the improved state afforded by post-snapshot quiescence technology, and therefore are not as useful when it comes time to restore the image backup chain to a restore storage.
The subject matter claimed herein is not limited to embodiments that solve any disadvantages or that operate only in environments such as those described above. Rather, this background is only provided to illustrate one example technology area where some embodiments described herein may be practiced.